Bioturbation by crab populations vis-à-vis sediment dispersal in Sagar Island, Hugli Estuary, India

Crab populations play a major role in net bioturbation of the coastal tracts of the Sagar Island, Hugli Estuary, India. The activities attest to scraping, churning, ingesting, burrowing substrate sediment for feeding, dwelling etc. Non-random distribution of bioturbated patches zone averaged for ten spatial zones indicates percent area bioturbation varying between 0.08% and 29.92%. No significant correlation exists between biological vis-a-vis geomorphological or sedimentological parameters as recorded under 12 chosen parameters varying in zones (Z1–Z10) and zone clusters (A, B and C). Bioturbated sediments, however, possess coarser mean size and better sorting than non-bioturbated sediments. We predict genetic link between biogenic coarsening of sediments and selective substrate aggradation as recorded in the zones (Z5 and Z6) of higher bioturbation (by percent area) over a decade (between 2008 and 2018). Onshore sediment transport in the study area appears to be influenced by the biogenic reworking and subsequent textural modifications of the substrate sediment. Fining landward trend is perceptible in the post-monsoon, non-bioturbated sediments that escaped seasonal storm reworking and biogenic reworking. Gradual inundation of the substrate is interpreted from the fining up sediment succession and the plant/animal traces preserved in them. This assay at Sagar Island predicts a constructive role of biogenic reworking by crab populations on sediment transport and substrate aggradation. Outcome of the present study may be applied to reconstruct sedimentation milieu in similar bioturbated and tidally influenced, retreating delta margin settings.